The present invention relates to means for switching between multiple signal sources, and in particular to a silent audio switch employing large switch arrays with built-in decoders in combination with a fast mixer circuit, to provide audio switching without causing audible pops.
In general, typical analog switchers such as those employed in video switcher apparatus to switch between as many as 32, or more, audio signal sources, generate switching spikes which can overload following audio stages, causing the well-known problem of switching noise, or "pops". The switching spikes are caused by switching during non-zero values of the input signal waveform, for example, by switching between two high quality audio signals of different magnitudes, and by the switch's drive signal coupling through its capacitance to the output. Since very high quality stereo audio is becoming increasingly popular in the television field, switching noise generated, for example, by switcher apparatus is totally unacceptable.
Typical of present schemes for eliminating such transient spikes when switching between high quality audio signals is the technique of increasing the switch driver's transition time to reduce the transient current through the switch capacitance. Thus, in switches where the input of the switch element is available, the transition time of the driver may be tailored to any value, making silent audio switching possible. Such switches generally employ a field effect transistor (FET) as the switching element in the respective audio signal path, and provide a relatively slow switch control signal, i.e., a ramp signal, to the FET. However, such schemes generally require a prohibitive amount of discrete circuitry since large switch array devices usually do not allow access to the control signal input of the respective switching element.
A more sophisticated scheme presently in use employs a matched pair of FETs for receiving respective audio signals, which are coupled together at their outputs to define a fast FET signal mixer configuration which mixes the two audio signals during a switching time interval. The switch drive voltage fed to the FETs is selectively controlled to extend the switching time interval over milliseconds, to eliminate capacitive feedthrough while performing what is hereinafter termed a "fast mix" between the audio signals. It follows that one signal is replaced by another signal without generating the undesirable transient spike. However, in a system such as used in switcher apparatus wherein as many as 16 stereo high quality audio signal inputs are common, this latter scheme is relatively cumbersome due to the fact that each of the corresponding large number of FET control signals are ramps of both increasing and decreasing magnitudes. This, in turn, necessitates the attendant complexity of multi-plexing a large number of ramp control signals. Further, the FETs must be matched to provide the optimum performance required of the mixer device. In addition, such scheme does not allow the advantageous use of present commercially available large switch matrices with built in decoding and drivers.
The present invention circumvents the problems and disadvantages generally found in typical analog audio switches while allowing the use of commercially available large switch matrices with decoding and switch drive facilities. This provides the added advantage of a relatively simple switching control circuit in systems employing large numbers of high quality audio input signals, particularly in a microprocessor controlled system. Thus the control circuitry is simplified while still retaining the silent switching provided, for example, by the soft switch technique obtained by use of a fast FET signal mixing circuit.
To this end, to obtain the above-mentioned advantages of the invention combination, at least two switch arrays are used to provide two identical matrices, with each array coupled to the multiple audio input signals. The arrays include multiple crosspoints, the number of which is dictated by the number of audio inputs and the array configuration selected. The outputs of the arrays are coupled to respective buses and thence to respective FET switches of a fast FET signal mixing circuit. The FET outputs, in turn, are connected together to provide the switch output.
Thus, it may be seen that two internal buses are used for each visible external bus. The invention contemplates switching crosspoints onto an "off-air", or "phantom", bus opposite an "on-air" bus, waiting for the transient spike to subside and then performing a fast mix between the two internal buses. In a microprocessor controlled scheme, the microprocessor remembers which internal bus was last written to, and which crosspoint was last selected on the opposite off-air bus. Thus, the invention contemplates a two-stage switching technique, utilizing the phantom array/bus configuration, wherein the new signal first is placed on the off-air array/bus and at a given time interval later, dictated generally by the switching speeds of the various components, is selected as the on-air array/bus via the FET signal mixing circuit. The prior on-air array/bus simultaneously is placed off-air.
The use of a phantom bus in combination with an on-air bus, to provide a two stage switching process which allows a transient spike and thus any associated "pop" to subside, further reduces the switch control complexity while minimizing the number of discrete components in the system. The technique also may be used to switch signals other than audio signals, i.e., video, control line, etc., signals, wherein switching transients are unacceptable.